Process and apparatus for recycling of synthetic plastics material containing gas

ABSTRACT

In a process for recycling of gas-containing synthetic plastics material, this material at first is comminuted, molten, degased and filtered. The degased synthetic plastics material is supplied in the same plant before its solidification for gas introduction, and in order to homogenize the gas content, the gas amount introduced into the synthetic plastics material per time unit is controlled in dependence from the synthetic plastics material amount supplied per time unit to the gas introduction, and preferably the pressure of the synthetic plastics material to which gas is to be added, is kept almost constant. The apparatus suitable for performing this process comprises an extruder (12,13) for plasticizing and degasing the synthetic plastics material, the outlet (15) thereof is connected by means of at least one line (16) to a mixer (25), to which also a device (29) for supplying gas is connected. A melt pump (23) for the synthetic plastics material is inserted into the line (16) and the amount of melt delivered by this pump is used as a command variable for the gas supply, for which a device (29) is provided that is controlled by a control unit (22) proportional to the speed of the melt pump (23) (FIG. 1).

This application is a continuation of U.S. patent application Ser. No.08/290,982, filed on Aug. 19, 1994, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a process for recycling of synthetic plasticsmaterial containing gas, for example foamed polystyrene, in a plant inwhich synthetic plastics material is molten, filtered, degased,granulated and mixed with gas again, this gas re-introduction beingperformed in the same plant as the granulation. Further, the inventionrelates to an apparatus for performing such a process.

The usual process steps when re-introducing gas into foamed syntheticplastics material, in particular polystyrene, consist in that thesynthetic plastics material is comminuted in a tearing and densifyingapparatus and is then plasticized in an extruder and de-gased. The soproduced plasticized, de-gased synthetic plastics material is worked upto granules in a granulating apparatus and is solidified thereby. Thegranulate forms a pourable homogeneous, de-gased mass which then isintroduced again into an extruder and is molten therein. During theextrusion which can be performed by means of extruders comprising one ormore extruder screw, a gas is fed in a controlled manner to the extruderwherein gas is introduced within the extruder into the syntheticplastics melt and is homogenized therewith, considering the necessaryholding time in the extruder. When doing this, the use of a pourablehomogeneous granulate is compulsory necessary because only in such amanner a continuous flow of the melt in the extruder and therefore acontinuous filling degree of the foaming gas in the melt can be ensured.

A similar process has become known from the document"Plastver-arbeiter", volume 42, 1991, number 6, pages 124, 125. Thisdocument suggests to mill packages of polystyrene and subsequently toheat this material in a roller extruder so that the material is de-gasedand plasticized. The thus obtained solidified final product, namelycristallized polystyrene, is then foamed again in an oxtruder.

These known processes have the disadvantage that two melting steps arenecessary which, as a rule, have to be carried out at different places,because the homogeneous introduction of the gas is a critical thing and,therefore, cannot be made everywhere. Therefore, as a rule, thesynthetic plastics material to be recycled is granulated there where anapparatus for comminuting and densifying the material is at disposal.The granulate is then sent to a plant where the material is providedwith a gas content again. This involves costs for transport andtransformation which lead to a high price of the recycled andgas-containing material so that the thus obtained recycling product canhardly be sold. The consequence is an environmental load by foamedsynthetic plastics material scrap or by improper processed syntheticplastics material. In this connection it is of detrimental influencethat in the past frequently such gases have been used as foaming gases(for example fluorocarbons) which are critical for environmental reasonsand, therefore, should not get into the atmosphere.

It is also known to melt and degas scraps of foamed thermoplasticsynthetic plastics material in an extruder comprising several worms. Inparallel to this extruder a further extruder is provided in whichthermoplastic particles, hence new material, are molten and mixed withgas. The outlets of the two extruders are combined to a mixing andcooling station to which a further extruder as well as a mould areconnected, in which the material is foamed. Finally, the foamed materialis cut to the desired shape or is granulated. For performing thisprocess, a considerable effort in apparatus and space is required andthe above described disadvantages can not fully be avoided by thisprocess, because the granulate of new material must at first be producedand then be stored so that the enthalpy is lost which was in the newmaterial until its granulation.

The invention has as its object to improve a process of the lastdescribed kind so that the recycling process of the gas-containingsynthetic plastics material can be effected quicker, simplier andrequiring considerably less effort in apparatus and space and withoutenvironmental load and without the use of new material. The inventionsolves this task in that the gas is fed to that synthetic plasticsmaterial which was de-gased and this before its granulation in suchplasticized condition of the synthetic plastics material whichoriginates from the melting process and the degasing step, and that forhomogenisation of the gas content the gas volume introduced per timeunit into the synthetic plastics material and the mixture volumeprocessed per time unit by the mixer are controlled proportionally tothe synthetic plastics material volume supplied per time unit to thegas-introduction step. For performing the inventive process, therefore,only one single plant is required and comminuting, plasticising,filtering, re-gasing and solidifying of the processed synthetic plasticsmaterial can be effected at the same place and, therefore, avoidingtransport costs. Within this, the known tearing and comminutingapparatus can be used, which must only be provided with a device for theintroduction of the gas and with a device for the subsequent mixing andsolidifying, in order to be able to perform the inventive process. Theplant expenses required for this are comparatively low and amortizealready within a short time by the saved transport costs and charge feesfor the extraneous device for re-introduction of the gas. The quality ofthe obtained final product is good, also with respect to homogenity ofthe foaming gas content, what is obtained by the inventive control ofthe gas introduction in dependence from the amount of flow of thesynthetic plastics material supplied to the gas introduction step. Afurther advantage of the inventive process consists in that theprocessed synthetic plastics material must be molten one single timeonly so that the second melting process required hereuntofore can besaved. This means also, that the synthetic plastics material is treatedmore gently because each melting process is combined with the danger ofa reduction of the molecule chain length of the synthetic plasticsmaterial. Further, when performing the inventive process, the syntheticplastics material must be solidified one single time only, and this--incontradiction to the initially described known processes--afterintroduction of the gas only.

In contradiction to the last described known process, within theinventive process only such synthetic plastics material is processedwhich was subjected to the recycling process. Therefore, the use of newmaterial and therefore the expenses for material, transport, storage andmelting connected therewith can be saved. Further, the inventive processrequires substantially less effort in apparatus, when compared with theknown processes.

The invention considers also that the synthetic plastics materialsubjected to the recycling process is not always of the same quality, inparticular with respect to the specific weight or, respectively, to thefoaming degree. The invention considers that, when material of lowerweight (higher foaming degree) is processed, the output of the apparatusused for the melting process will decrease. As a consequence thereof,the synthetic plastics material amount supplied per time unit to the gasintroduction step, decreases. If this volume measured per time unit isused in the sense of the invention as the command variable for the gasamount used per time unit for being added to the synthetic plasticsmaterial, the gas content for the final recycling product remains atleast substantially constant with what is desired with respect to thefurther processing of this final product. The homogenity of this finalproduct is also enhanced by the proportional readjustment of the mixtureamount worked up per time unit by the mixer, in dependence from thementioned command variable, because undermixing as well as overmixingare avoided and thus the cell structure of the regenerate obtained asthe final product is kept at least substantially constant.

Since as a rule it is desired to operate the plant with an output ashigh as possible or, respectively, as far as possible to fully utilizethe capacity of the present plant, according to a preferred embodimentof the inventive process, the volume of the synthetic plastics materialsupplied per time unit to the gas introduction step is kept within apredetermined interval, preferably near the maximum capacity of theplant used. This can easily be obtained by operating the plant elementsused for plasticizing and de-gasing of the supplied synthetic plasticsmaterial quicker or, respectively, with an increased output.

As a rule, a pump for the synthetic plastics material flow supplied tothe gas introduction step is inserted between those plant elements thatserve for plasticizing the supplied synthetic plastics material, andthose plant elements that serve for the re-introduction of the gas.According to a preferred embodiment of the inventive process, thesupplied synthetic plastics material is comminuted and then isplasticized and de-gased by means of a screw, that the so obtainedde-gased melt is supplied by means of a melt pump, preferably a gearpump, to a mixer to which also the gas used is supplied, and that thesynthetic plastics material amount delivered per time unit by the meltpump is measured and is used as the command variable for the gas supply,and that the pressure of the synthetic plastics material on the suctionside of the melt pump is measured and is kept by varying the speed ofthe melt pump and/or the speed of the worm within a predeterminedinterval, preferably at a value that is as constant as possible.Therefore, the melt pressure before the melt pump is kept almostconstant by adjusting the delivery of the plasticizing screw or the meltpump, and by proportional readjustment of the gas supply in dependenceon the delivery per time unit of the melt pump used as the commandvariable, the desired constant gas content in the final product producedby the mixer is obtained. Keeping the pressure of the material suppliedto the melt pump within a predetermined interval ensures that the meltpump does not make idle strokes, and that therefore the volume deliveredis exactly proportional to the run of the melt pump, in particular tothe speed of the gear pump. Further, in such a manner the plant caneasily be operated near its maximum capacity.

Within the spirit of the invention, it is particularly favourable to addthe gas to the flow of the synthetic plastics material already beforeits introduction into the mixer, in order to obtain a certain mixturebetween the synthetic plastics material and the foaming gas alreadybefore the synthetic plastics material or, respectively, the gas entersthe mixer.

For a uniform absorption of the gas, the melt pressure behind the meltpump is of importance. According to a further embodiment of theinvention, therefore, the process is conducted so that when the pressureof the synthetic plastics material melt supplied by the melt pumpdeviates from a predetermined desired value, the viscosity of themixture prepared by the mixer is correspondingly changed by coolingand/or heating, preferably until the desired value is reached.

As a rule, pentane is used for the gas that is again introduced. Inorder to avoid an explosion in the plant section behind the degasingstep, within the spirit of the invention the vacuum is controlled thatis used for degasing the synthetic plastics material.

The inventive apparatus for performing the inventive process starts froma plant having a comminuting means for the synthetic plastics materialto be processed, to which means a motor-driven screw for plasticizingthis material is connected, a de-gasing means for the synthetic plasticsmaterial being related to that screw. Starting therefrom, the inventiveapparatus is characterized in that the outlet of the screw is connectedby means of at least one line to a mixer to which also a device forsupplying gas is connected, and that to at least one of these lines adevice for measuring the synthetic plastics material amount flowing pertime unit in this line is connected, which device is connected to acontrol unit for the gas supply to the mixer and for the speed of themotor of the screw. In such a manner the inventive process can beperformed with low constructional effort. Since, however, the pressurein the outlet of the screw and, therefore, in the line leading to themixer alone will frequently not be sufficiently high in order toreliably supply the plasticized synthetic plastics material into themixer, according to a preferred embodiment of the inventive apparatus, amelt pump, in particular a gear pump, for supplying the syntheticplastics material melt is inserted into the line leading from the screwto the mixer, a measuring device for the speed of this melt pump beingprovided, noting that to this line a pressure sensor for the pressure atthe suction side of this melt pump is connected, which pressure sensor,as well as the measuring device, are connected to the control unit. The,suitably electronic, control unit calculates the measuring resultsobtained from the pressure sensor and from the measuring device andcontrols in correspondence to these measuring results the gas supply tothe synthetic plastics material melt in such a manner that the gascontent thereof is kept at least substantially constant so that ahomogeneous final product is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention can beseen from the description of two embodiments of the inventive apparatuswhich are schematically shown in the drawings wherein:

FIG. 1 is a schematic view of an apparatus of the present invention; and

FIG. 2 is a schematic view of an apparatus of another preferredembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the embodiment of FIG. 1 the plant 1 for carrying out theprocess has a comminuting apparatus 2 formed by a tearing and condensingmachine, to which the synthetic plastics material 3 to be recycled, inparticular foamed polystyrene, for example STYROPOR (registeredtrademark) is supplied by means of a conveyor 4. The synthetic plasticsmaterial 3 falls from above into a receptacle 5 of the comminutingapparatus 2 and a tool 6 for comminuting and/or mixing of the syntheticplastics material 3 rotates in the bottom region of the comminutingapparatus 2 around a vertical axis 7, the tool 6 being driven by a motor8. The tool 6 is provided with knives 9 acting on the synthetic plasticsmaterial 3, which knives comminute and mix the material 3 which rotatesin the receptacle 5 in form of a mixing thrombe 10. An outlet opening 11is disposed in the sidewall of the receptacle 5 at the level of the tool6, to which opening the housing 12 of a screw 13 is connected in radialdirection. The screw is driven for rotation by a motor 14 disposed atthat end of the screw which does not face the receptacle 5. By means ofthe screw 13 the synthetic plastics material supplied from thereceptacle 5 is plasticized and is pressed in plasticized or moltencondition at the outlet 15 of the housing 12 into a line 16. The housing12 is further provided with lateral openings through which gases carriedalong by the melt conveyed by the worm 13 can escape into a degasingdevice 17 via one or more lines 18. Suitably, these gases are collected,cleaned (if necessary) and supplied to a recycling process.

At least one filter means 19 is inserted into the line 16 by whichfilter the synthetic plastics material melt is freed from impuritiescarried along. In front of the filter 19, when seen in flowing directionof the synthetic plastics material melt (arrow 20), a pressure sensor 21is connected to the line 16, which sensor is also connected to a controlunit 22 by which the entire plant 1 is monitored and is controlled inthe sense of an optimal operation condition. To this control unit 22also the motor 14 is connected. A melt pump 23 formed by a gear pump isinserted into the line 16 behind the filter 19, which pump is driven bya motor 31 and supplies the synthetic plastics material through the line16 into the housing 24 of a mixer 25 which may be formed by a static ordynamic mixer. The embodiment illustrated shows a dynamic mixer whichcomprises a mixing element, for example a mixing worm 26 bearinglysupported for rotation within a housing 24 and driven for rotation by amotor 27. Instead of such a dynamic mixer also a static mixer can beused which comprises baffles in the housing 24 by which the suppliedsynthetic plastics material is mixed. In the mixer 25 also the gas isintroduced again into the synthetic plastics material or is worked upinto this plasticized or melt-like synthetic plastics material, whichconstitutes an intermediate product only and is directly supplied by thepump 23 to the mixer 25 without adding any gas-containing syntheticplastics material or new synthetic plastics material. For this, afurther pressure sensor 28 and a means 29 for the addition of gas to thesynthetic plastics material melt are connected to the line 16 betweenthe melt pump 23 and the mixer 25. A further pressure sensor 30 isconnected to the line 16 between the filter 19 and the pump 23. Themeans 29 for supplying gas has a proportioning device 32 comprising apump for the propelling gas to be added to the synthetic plasticsmaterial melt, which proportioning device 32 is driven by a motor 33 andis supplied with a foam producing gas or "foaming agent", for examplepentane which is liquid at atmospheric pressure, from a foaming gassource 34. All these motors 14, 27, 31, 33 as well as the pressuresensors 21, 28, 30 and the filter 19 are connected to the control unit22. Further, a motor 35 for a granulating means 36 is connected to thiscontrol unit 22, to which granulating means the synthetic plasticsmaterial to be granulated is supplied from the mixer 25 via an extruderhead 37. Before the extruder head 37 rotates at least one knife 38driven by a motor 35 and the cut-off synthetic plastics materialparticles gather within a granulator housing 39 where they are cooledand conveyed off through an outlet 39' in direction of the arrow 40.

The synthetic plastics material to be provided with gas again issupplied exclusively from the extruder formed by the housing 12 and thescrew 13, however, of course a plurality of such extruders may in commonbe connected to the line 16. Therefore, no addition of new materialtakes place to that synthetic plastics material which is conveyed by thescrew 13 into the line 16, unless new material together with othersynthetic plastics material is conveyed by the conveyor 4 into thecomminuting means 2. The latter may be the case if scraps obtained fromthe production of new foamed synthetic plastics material are processed,for example rests of foamed synthetic plastics material blocks or thelike.

The entire plant 1 is controlled by the control means 22 so that thefilling degree of the foaming gas in the synthetic plastics materialgranulate that is conveyed off through the outlet 39' remains at leastsubstantially constant, even if the quality of the synthetic plasticsmaterial 3 supplied by the conveyor 4 changes. For this, the plantcomprises a device 43 for measuring the synthetic plastics materialamount flowing per time unit in the line 16, which device 43 isconnected to the control unit 22. Since the pressure sensor 30 and thecontrol unit 22 ensure that the pressure of the synthetic plasticsmaterial supplied to the gear pump 23 via the line 16 is kept within apredetermined pressure interval so that the gear pump 23 is alwayscompletely filled, however also an unstable compression of the syntheticplastics material is avoided, the synthetic plastics material massconveyed by the gear pump 23 is proportional to the speed of revolutionof the gear pump 23. In the simpliest case, this speed can be measuredby means of a revolution counter 44 forming the means 43 and can betransmitted to the control means 22. In order to consider also changesof the quality of the synthetic plastics material supplied to the entireplant 1, the pressure sensor 30 is provided which senses the pressure infront of the gear pump 23. Thus, if for example the screw 13 forming anextruder takes up synthetic plastics material 3 of lower weight(material having a higher foaming degree), the synthetic plasticsmaterial mass delivered per time unit through the outlet 15 into theline 16 decreases. As a consequence, also the pressure in front of thegear pump 23 will decrease what is transmitted to the control unit 22 bythe pressure sensor 30. The control unit causes that the screw 13 isdriven quicker by the motor 14 and/or that the melt pump 23 is driven bythe motor 31 with a reduced speed. If the speed of the gear pump 23 ischanged, the revolution counter 44 of the means 43 causes the controlunit 22 at the same time to proportionally adjust a gas pump 45 providedin the dosing device 32. In order to keep the pressure in front of thegear pump 23 sensed by the pressure sensor 30--as mentionedabove--within a predetermined interval so that it is avoided that thespacewidths of the gear pump 23 are not completely filled with syntheticplastics material or that the synthetic plastics material isadditionally compressed by the melt pump 23, mainly the operationalspeed of the plasticizing screw 13 is correspondingly controlled via thecontrol unit 22 by controlling the speed of the motor 14. In additionthereto--in particular if the mentioned speed control of the motor 14should not be sufficient--also the speed of the motor 31 of the gearpump may be controlled correspondingly by the control unit 22. Thiscauses that the flow delivered per time unit by the gear pump 23 ischanged what is transmitted via the revolution counter 44 to the controlunit 22 which causes a corresponding proportional control of the gasdosing in the device 29, for example in a simple manner by controllingthe operational speed of the gas pump 45 by means of the motor 33.Further, the delivery pressure of the gear pump 23 is measured by thepressure sensor 28 connected to the line 16 behind the gear pump 23.Since this pressure is of importance for the gas take up of the melt,the control unit 22 keeps this pressure as constant as possible. Forthis, at first the motor 27 of the mixer is so influenced by the controlunit 22 that the mixer 25 takes up just that volume of the syntheticplastics material-gas-mixture that is supplied to it from the gear pump23 or, respectively from the gas pump 45. In addition thereto, theresistance in the mixer 25 may be influenced by changing the viscosityof the synthetic plastics material within the mixer 25. For this, aplurality of heating zones or, respectively, cooling zones 47 aredisposed on the periphery of the housing 46 of the mixer 25, which zonestogether build up a heating- and cooling-cascade control and may beswitched in individually or in any desired combination by the controlunit 22 in dependence of the constant pressure in that section of theline 16 that is disposed behind the gear pump 23, which pressure servesas a command variable.

As it is shown in the drawing, the line 41 via which the foaming gas isadded to the melt, ends into the line 16 so that in that section of theline 16 that is disposed between the line 41 and the mixer 25, already apre-mixing of the synthetic plastics material and the foaming gas takesplace. However, the line 41 may also be connected directly to thehousing of the mixer 25. Suitably, a non-return valve (not shown) isinserted into the line 41.

As it is further shown in the drawing, the two screws 13, 26 areprovided on their ends facing the motors 14, 27 with volutions 42conveying in inverse direction, so that a sealing for the driving shaftsdriven by the motors 14, 27 is formed.

The pressure sensor 21 serves for sensing the pressure of the syntheticplastics material melt directly at the outlet 15 and in front of thefilter 19. Thereby, soiling of the filter 19 can be monitored and ascreen exchange or screen backwashing step can be initiated in time, ifthe pressure in the line 16 in front of the filter 19 exceeds apredetermined value. These steps are also initiated by the control unit22. Suitable filter means 19 which enable one to change a screen or toclean a screen, for example by backwashing, without substantialimpairment of the synthetic plastics material pressure in the line 16behind the filter 19, are known.

Of course it is also possible to connect more than one comminuting means2 and extruders 12, 13 connected thereto by a plurality of lines 16 to acommon mixer 25. In an analogous manner it would also be possible toconnect one single extruder 12, 13 by means of a plurality of parallellines 16 to the mixer 25.

It would also be possible to introduce the plasticized syntheticplastics material via the line 16 directly into the mixer 25, thuswithout that this material is conveyed by a melt pump 23. This, however,requires that the flow of the synthetic plastics material 16 in thisline can be monitored in a suitable manner, for example by means of ausual flow meter. By the said reasons also the pressure in the line 16must be monitored. Control of the gas supply or of the speed of the worm13 or of the speed of the mixer 25 is done by the control unit 22 in ananalogous manner to that described above.

Within the embodiment according to FIG. 2, a vacuum control means 48 isconnected to the housing 12 of the worm 13, said means 48 transmits themeasured vacuum to the control unit 22. Thereby the function of thedegasing device 17 is monitored so that any danger of explosion of thepentane used for the gas addition is avoided. Further, the granulatingdevice 36 is not directly connected to the granulating head 37 or itsnozzle 49, but by intermediary insertion of a flow cooling device 50, inwhich the nozzle 49 is disposed. The synthetic plastics material cordproduced by it is conveyed within a cooling bath in the cooling device50 and enters the granulating device 36 only after having passed theflow cooling device 50, and in the granulating device 36 there aredisposed rotating knives which cut the granulate particles in a wellmanner from the synthetic plastics material cord. The granulate leavesthe granulating device 36 through the outlet 39' in direction of thearrow 40. Also the conditions within the flow cooling device 50 may beinfluenced by the control unit 22 in any desired manner.

We claim:
 1. In a process for recycling of synthetic plastics materialcontaining gas in which the synthetic plastics material to be recycledis molten and then is filtered, de-gassed and granulated, wherein theimprovement comprises the steps of:plasticizing and de-gassing thesynthetic plastics material within a first housing containing a firstscrew-type means; removing the de-gassed, plasticized synthetic plasticsmaterial from the first housing through an outlet; introducing a foamingagent into the de-gassed, plasticized synthetic plastics material beforethe granulation and after the de-gassing step and after the removal ofthe material from the first housing; mixing and homogenizing thede-gassed, plasticized synthetic plastics material with the foamingagent within a second housing containing a second screw-type means toproduce a mixture thereof, the second housing and second screw-typemeans being separate from and operable independently of the firsthousing and screw-type means; wherein for homogenization of the foamingagent content the volume of foaming agent introduced per time unit intothe de-gassed, plasticized synthetic plastics material and the volume ofthe mixture processed per time unit are controlled proportionally to thesynthetic plastics material volume supplied per time unit at theinroduction of the foaming agent; and finally only then extruding themixture from the second housing and granulating the same.
 2. Processaccording to claim 1, the volume of the synthetic plastics materialsupplied per time unit to the foaming agent introduction being keptwithin a predetermined interval.
 3. Process according to claim 2, thevolume of the synthetic plastics material supplied per time unit to thefoaming agent introduction being kept near the maximum capacity of aplant in which the process is performed.
 4. Process according to claim1, the synthetic plastics material to be recycled being comminuted andthen is plasticized and de-gassed by means of a screw, the so obtainedmelt being filtered and supplied by means of a melt pump to a mixer, towhich also the foaming agent to be added is supplied, and the syntheticplastics material amount delivered per time unit by the melt pump beingmeasured and used as the command variable for the foaming agent supply,and the pressure of the synthetic plastics material on the suction sideof the melt pump being measured and kept by varying one of the speed ofthe melt pump and the speed of the screw within a predeterminedinterval.
 5. Process according to claim 4, the pressure of the syntheticplastics material on the suction side of the melt pump being kept byvarying one of the speed of the melt pump and the speed of the screw ata value that is substantially at a constant.
 6. Process according toclaim 4, the foaming agent being added to the flow of the syntheticplastics material already before its introduction into the mixer. 7.Process according to claim 4, when the pressure of the syntheticplastics material melt delivered by the melt pump deviates from apredetermined desired value, the viscosity of the mixture worked up bythe mixer being correspondingly varied by cooling and/or heating untilthis desired value is reached.
 8. Process according to claim 1, whereina vacuum is used for de-gassing the synthetic plastics material, saidvacuum being selectively controllable.
 9. An apparatus for recyclingsynthetic plastics material comprising:comminuting means for comminutingthe synthetic plastics material to be recycled, said comminuting meansbeing connected to at least one screw driven by a motor for plasticizingthe material; a de-gassing and filtering device for de-gassing andfiltering the synthetic plastics material, the de-gassing and filteringdevice being related to said screw wherein the outlet of the screw onlyis connected by at least one line to a mixer to which also a device forintroducing a foaming agent into the plasticized and filtered syntheticplastics material is connected; and a device for measuring theplasticized synthetic plastics material amount flowing per time unitbeing connected to said at least one line, which measuring device beingconnected with a control unit for the foaming agent supply to the mixerand for the speed of the motor of the screw, the mixer being connectedto a granulation device for granulating the mixture of the plasticizedsynthetic plastics material and of the foaming agent.
 10. Apparatusaccording to claim 9 further comprising:a melt pump for supplyingsynthetic plastics material which is inserted into the line leading fromthe screw to the mixer; a measuring device for measuring the speed ofthe melt pump being provided; and a pressure sensor for measuring thepressure on the suction side of this melt pump being connected to thisline this pressure sensor and the measuring device being connected tothe control unit.
 11. Apparatus according to claim 10, said measuringdevice comprising a revolution speed meter.
 12. Apparatus according toclaim 9, the device for the foaming agent supply being connected bymeans of a line to the line supplying the synthetic plastics material tothe mixer.
 13. Apparatus according to claim 10, a filter of thede-gassing and filtering device being inserted before the melt pump. 14.Apparatus according to claim 13, a further pressure sensor beingconnected in front of the filter to the line supplying the syntheticplastics material to the melt pump, said pressure sensor further beingconnected to the control unit.
 15. Apparatus according to claim 10, apressure sensor for the pressure of the synthetic plastics materialsupplied from the melt pump to the mixer being connected to the line,said pressure sensor further being connected to the control unit. 16.Apparatus according to claim 15, the mixer being provided with at leastone heating or cooling zone connected to the control unit.
 17. Apparatusaccording to claim 9, a vacuum control device being connected to thehousing of the screw in the region of the de-gassing device.
 18. Aprocess for recycling synthetic plastics material which contains gas,comprising the steps of:(a) plasticizing the synthetic plastics materialwithin a first housing containing a first screw-type means; (b)de-gassing the synthetic plastics material within the first housing; (c)removing the de-gassed, plasticized synthetic plastics material from thefirst housing through an outlet; (d) filtering the synthetic plasticsmaterial; (e) after said de-gassing and filtering, introducing a foamingagent into the thus obtained plasticized synthetic plastic material,said foaming agent being introduced into the synthetic plastics materialas a liquid; (f) introducing the de-gassed, filtered plasticizedsynthetic plastic material and foaming agent into a second housingcontaining a second screw-type means; (g) mixing and homogenizing thede-gassed, filtered plasticized synthetic plastic material and foamingagent within the second housing; (h) controlling the volume of thefoaming agent introducing per time unit into the synthetic plasticsmaterial and the volume of said mixture processed per time unit so thatthese volumes are maintained at a predetermined proportion forhomogenizing the foaming agent content in the mixture; (i) extruding andgranulating the mixture containing the foaming agent; and (j) whereinthe introduction of the foaming agent being performed before granulatingthe mixture in a still plasticized condition of the synthetic plasticsmaterial originating from the plasticizing of this material and afterthe removal of the plasticized synthetic plastics material from thefirst housing.
 19. A process for recycling synthetic plastics materialwhich contains gas, comprising the steps of:(a) plasticizing thesynthetic plastics material within a first housing containing a firstscrew-type means; (b) de-gassing the synthetic plastics material withinthe first housing; (c) removing the de-gassed, plasticized syntheticplastics material from the first housing through an outlet; (d)filtering the synthetic plastics material; (e) after said de-gassing andfiltering, introducing a foaming agent into the thus obtainedplasticized synthetic plastic material, said foaming agent beingintroduced into the synthetic plastics material as a gas; (f)introducing the de-gassed, filtered plasticized synthetic plasticmaterial and foaming agent into a second housing containing a secondscrew-type means; (g) mixing and homogenizing the de-gassed, filteredplasticized synthetic plastic material and foaming agent within thesecond housing; (h) controlling the volume of the foaming agentintroducing per time unit into the synthetic plastics material and thevolume of said mixture processed per time unit so that these volumes aremaintained at a predetermined proportion for homogenizing the foamingagent content in the mixture; (i) extruding and granulating the mixturecontaining the foaming agent; and (j) wherein the introduction of thefoaming agent being performed before granulating the mixture in a stillplasticized condition of the synthetic plastics material originatingfrom the plasticizing of this material and after the removal of theplasticized synthetic plastics material from the first housing.